D01 Answer

#include <stdio.h>
#include <stdlib.h>

/* Puzzle D01 -- fill an N by M integer array with zeros and print it out. */

int main(int argc, char *argv[])
{	
	const int Nrows = 3, Mcols = 5 ;
  int x[Nrows][Mcols] ;
  
  int count = 0; /* Values to copy to the array */
  
  int r, c;  /* row and column indexes for the array */
  
  /* Fill the array with ascending integers */
  for ( r=0; r<Nrows; r++ )
    for ( c=0; c<Mcols; c++ )
      x[r][c] = count++ ;
    
  /* Print out the array */
  for ( r=0; r<Nrows; r++ )
  {
    for ( c=0; c<Mcols; c++ )
      printf("%2d ", x[r][c] );
    printf("\n");  
  }
  
  printf("\n");
  system("PAUSE");	
  return 0;
}

Comments: Arrays are stored in main memory in row-major order, which means that the elements of a row are stored in sequence in memory.

The above program stores integers in order into the array. Conceptually the above 3x5 array looks like this:

However, in main memory the elements are stored like this (assume that memory addresses increase to the right):

Because of this, code that accesses every element of a 2D array should be written, as follows, so that elements are accessed in sequence through memory:

  for ( r=0; r<Number_Of_Rows; r++ )
  {
    for ( c=0; c<Number_Of_Columns; c++ )
    {  
       ... do something with element[r][c] ...
    }
  }